A strong bimetal-support interaction in ethanol steam reforming

The metal-support interaction (MSI) in heterogeneous catalysts plays a crucial role in reforming reaction to produce renewable hydrogen, but conventional objects are limited to single metal and support. Herein, we report a type of RhNi/TiO2 catalysts with tunable RhNi-TiO2 strong bimetal-support interaction (SBMSI) derived from structure topological transformation of RhNiTi-layered double hydroxides (RhNiTi-LDHs) precursors. The resulting 0.5RhNi/TiO2 catalyst (with 0.5 wt.% Rh) exhibits extraordinary catalytic performance toward ethanol steam reforming (ESR) reaction with a H2 yield of 61.7%, a H2 production rate of 12.2 L h−1 gcat−1 and a high operational stability (300 h), which is preponderant to the state-of-the-art catalysts. By virtue of synergistic catalysis of multifunctional interface structure (Rh-Niδ−-Ov-Ti3+; Ov denotes oxygen vacancy), the generation of formate intermediate (the rate-determining step in ESR reaction) from steam reforming of CO and CHx is significantly promoted on 0.5RhNi/TiO2 catalyst, accounting for its ultra-high H2 production.

3. Page 5, section Catalytic performance and kinetic analysis toward ESR: Based on the cation of Supplementary Figure 6, it could be supposed that the time on stream for catalytic tests performed at temperatures of 350 and 400 °C equals 1.5 hours. Why this time is so short? If the Authors run the catalytic tests longer, there is possible that the deactivation of some catalysts could be observed. 4. Page 8, section Catalytic performance and kinetic analysis toward ESR: 'Furthermore, we performed kinetic tests of CO and CH4 steam reforming reaction to simulate the reaction rate of CO and CHx intermediates during ESR reaction (Fig. 1e). ' The Authors indicate that performed catalytic tests. As it is well known these kinds of tests should be carried out at low ethanol conversion. Thus, the Authors should indicate in Figure in Supplementary materials that the conversion of ethanol during these tests was lower than 10%. 5. Page 22: Taking into account the amount of research carried out for the catalysts studied in the ESR process, the conclusions are rather short and poor and must be rewritten.

Reviewer #1
Remarks to the Author: 0.5RhNi/TiO2 catalyst was synthesized and illustrated to be effective ESR. Various analysis including DFT calculations were performed to understand the performance of the synthesized catalysts. Most of the work was carried out carefully and the results seem to be significant. However, I will only recommend its publication after resolving the following concerns.
(1) Hydrogen production of ESR is dependent on the ration of S/C. on p.5, it states as 3 but in figures, such as Fig.1, it is 6. Which one is correct? A direct comparison in Table 2 of supporting information cannot be made directly if the current work is 6.
Author reply: Thank you very much for this comment. On Page 5, the value of 3 is the steam/carbon ratio (S/C), where one ethanol molecule provides two carbon atom and thus the ratio is 3. However, in the caption of Fig. 1, the value of 6 denotes the molar ratio of water and ethanol for the inlet gas. Both values are correct with different expressions. Thus, in Table 2 of supporting information, the S/C ratio is 3. According to the suggestions above, in order to avoid misunderstanding, we adopted a unified form of steam/carbon ratio (S/C) of 3 in the revised manuscript.
• Page 5, Line 18: rephrase: "The ESR reaction was performed in a fixed-bed reactor with steam/carbon (S/C) ratio of 3." Author reply: Thank you for this comment. To clearly identify the rate-determining step, we measured the apparent activation energy and reaction order of ethanol dehydrogenation, acetaldehyde decomposition, steam reforming of CO (or CH4) based on kinetics studies in the revised manuscript (Supplementary Figure 14). In addition, according to the report mentioned above, we also calculated the reaction energy barrier for ethanol dehydrogenation (CH bond cleavage) and acetaldehyde decomposition (CC bond cleavage) through DFT calculations ( Supplementary Fig. 53). Corresponding discussions have been added in the revised manuscript.
• Page 8, Line 17: rephrase: "Furthermore, we performed kinetic tests of ethanol dehydrogenation, acetaldehyde decomposition, steam reforming of CO (or CH4) to simulate the reactivity of CH bond cleavage, CC bond cleavage, CO or CHx transformation during ESR reaction. As shown in Supplementary Fig. 14a, the activation energy of elementary reaction gives the following sequence: ethanol dehydrogenation (44.54 kJ/mol) < acetaldehyde decomposition (50.45 kJ/mol) < CO steam reforming (89.46 kJ/mol) < CH4 steam reforming (101.26 kJ/mol), which indicates that the cleavage of CH and CC bonds is facile whilst the transformation of CO and CHx is involved in the rate-determining step. This result is further demonstrated through a more significant concentration-dependence of reaction order for CO and CH4 relative to ethanol and acetaldehyde: ethanol (0.46) < acetaldehyde (0.62) < CO (1.18) < CH4 (1.23) ( Supplementary Fig. 14b)." • Page 20, Line 9: rephrase: "In addition, we also calculated the reaction energy barriers for ethanol dehydrogenation and acetaldehyde decomposition through DFT calculations  (4) Eq(2) and Eq(4) did not include water in the calculation but water provides half of H atoms to form H2 in a perfect ESR.
Author reply: Thank you for this comment. Actually, this issue (half of hydrogen provided by water) has been considered and included in Eq (2): Author reply: Thank you for this comment. We re-tested the catalytic performance of Ni/TiO2, 0.5RhNi/TiO2, Rh/Ni and Rh/TiO2 samples at a higher weight hourly space velocity (WHSV: 28 h 1 ) and gas hourly space velocity (GHSV: 16700 h 1 ) by reducing the catalyst dosage from 0.15 g to 0.05 g, and other reaction conditions remained unchanged ( Supplementary Fig. 8).
Corresponding discussion has been added in the revised manuscript.
• Page 6, Line 7: rephrase: "Furthermore, we also tested the ESR reaction at a higher WHSV (21 h 1 ) and GHSV (16700 h 1 ) at 400 C (ethanol conversion less than 70%), and the 0.5RhNi/TiO2 catalyst still displayed the optimal hydrogen yield and relatively low CH4 and CO yields ( Supplementary Fig. 8)." Author reply: Thank you for this comment. According to this comment, we carried out time on stream (TOS) tests for 0.5RhNi/TiO2, Ni/TiO2, Rh/Ni and Rh/TiO2 at 400 °C, respectively, and the results were shown in Supplementary Fig. 9 in the revised SI. The related discussion has been added in the revised manuscript.
• Page 6, Line 12: rephrase: "In addition, the time on stream (TOS) tests for Ni/TiO2, Rh/Ni, 11 Rh/TiO2 and 0.5RhNi/TiO2 were carried out at 400 °C, respectively. After reaction for 40 h, the ethanol conversion over Ni/TiO2, Rh/Ni, and Rh/TiO2 decreases from 100% to 91.33%, 58.54% and 74.16%, respectively ( Supplementary Fig. 9). In contrast, both the ethanol conversion and hydrogen yield in the presence of 0.5RhNi/TiO2 catalyst remain stable within 300 h (Fig. 1d)." (4) Page 8, section Catalytic performance and kinetic analysis toward ESR: 'Furthermore, we performed kinetic tests of CO and CH4 steam reforming reaction to simulate the reaction rate of CO and CHx intermediates during ESR reaction (Fig. 1e).

' The authors indicate that performed catalytic tests. As it is well known these kinds of tests should be carried out at low ethanol conversion. Thus, the Authors should indicate in Figure in
Supplementary materials that the conversion of ethanol during these tests was lower than

10%.
Author reply: Thank you for this comment. For the kinetic tests, we did proceed at a low conversion of ethanol, CO and CH4 (less than 10%). The relevant description has been added in the caption of Figure 1  Author reply: Thank you for this comment. According to this suggestion, we have improved the conclusion in the revised manuscript.
• Page 23, Line 4: rephrase: "In summary, we report a RhNi/TiO2 catalytic system with welldefined SBMSI towards ESR reaction. The obtained 0.5RhNi/TiO2 catalyst gives an exceptional hydrogen production (H2 yield: 61.7%) and catalysis stability (300 h) at a relatively low temperature (400 C). The microscopic fine-structure of RhNi/TiO2 was studied by STEM, CO-DRIFT and XAFS, in which the RhNi bimetallic nanoparticle with a reversible TiO2 coating exhibited a multiple electron transfer pathway at the interfacial active sites (Rh-Ni δ -Ov-Ti 3+ ). A comprehensive investigation including in situ spectroscopic characterizations, operando pulse experiments, kinetics studies and DFT calculations substantiates that the ESR reaction in the presence of 0.5RhNi/TiO2 catalyst follows a CO/CHx-mediated reforming process rather than the acetate path. This bimetal-support interface (Rh-Ni δ -Ov-Ti 3+ ) plays a decisive role in steam reforming of CO and CHx originating from ethanol dissociation, which is involved in the rate-determining step of ESR reaction. The modulated geometric and electronic structure of interfacial active sites resulting from SBMSI reduce the binding ability of species with COO  structure (CO2, carbonate or formate). This facilitates the generation and 13 transformation of formate intermediate from CO/CHx reforming processes, which ensures a prominent hydrogen production rate and catalytic stability. The well-defined SBMSI demonstrated in this work can be extended to other structure-sensitive reactions involving multiple reaction substrates."

Reviewer #1 (Remarks to the Author):
Authors made efforts in the revision to address the concerns by the referees. Although improvements are made, I will only recommend its publication after resolving the following concerns. 1) Reaction mechanism of ESR is extremely complicated. Authors claimed in the response that they calculated the reaction energy barrier for ethanol dehydrogenation according to the report mentioned (i.e. recommended reference). There is an issue: the authors only choose to calculate the barrier for CH3CH2OHCH3CH2O, which is not the case shown in the reference. At least some justification is needed on why they choose only this dehydrogenation when three possible pathways are feasible and needed to study before concluding it is the one. Furthermore, in recent bimetallic studies, the relative energetics of reaction pathways may change due to alloying (is especially true in authors' work) and brings some extra challenge to the computational studies of ESR mechanism, see Wu, et al, J. Phys. Chem. 126(2022)21650, and therefore caution should be made when using limited DFT studies.
2) Experimental measurements shown in supplementary Figure 14 are great. When they are correlated wrongly with the elementary reactions, conclusion can mislead readers. Therefore, a link between the observations and the DFT results needs to be established carefully so that the results of the work are impactful. Author reply: Thank you very much for this comment. Based on the product distribution and operando characterizations (DRIFTS spectra and pulse experiment), important reaction intermediates (e.g., CH3CH2O, CH3CHO, CH3 and CO) were captured, which verified the ESR reaction pathway over the RhNi/TiO2−x catalyst. Ethanol undergoes dehydrogenation to acetaldehyde, followed by acetaldehyde decomposition to CO and CHx. Subsequently, the resulting CO and CHx as key intermediates react with H2O to produce CO2 and H2. Thus, we speculate experimentally that ethanol dehydrogenation involves the cleavage of O−H (in 2 hydroxy) and C−H (in methylene rather than methyl) bonds to produce CH3CO*. According to this comment, we supplemented the ethanol dehydrogenation steps on Rh1Ni7/TiO2−x and Ni8/TiO2−x systems through DFT calculations. In addition, comparative studies on ESR reaction mechanism were also conducted with mono-metal system (Ni8/TiO2−x), and the corresponding discussions have been added in the revised manuscript and Supplementary Information.  Table 8 and Supplementary Note 22), the optimal ethanol dehydrogenation path in Ni8/TiO2−x and Rh1Ni7/TiO2−x systems follows CH3CH2OH → CH3CH2O* → CH3CHO* → CH3CO*, and then CH3CO* undergoes C−C bond breaking to produce CH3* and CO, which is consistent with the product distribution and operando characterizations (DRIFTS spectra and pulse experiment). Compared with Ni8/TiO2−x, the energy barriers for ethanol dehydrogenation and C−C bond cleavage decrease from 1.65 and 1.31 eV to 1.03 and 1.17 eV on Rh1Ni7/TiO2−x, respectively." • Page 21, Line 14: rephrase: "In contrast, the formate formation from C/CH fragment shows an energy barrier of 2.36 and 2.79 eV on Rh1Ni7/TiO2−x and Ni8/TiO2−x catalysts, respectively, much larger than that of ethanol dehydrogenation (1.03 and 1.65 eV) and acetaldehyde decomposition (1.17 and 1.31 eV), indicating that the transformation of CO and CHx is the crucial step, in accordance with the experimental results. Especially, the lower reaction energy barriers on Rh1Ni7/TiO2−x relative to Ni8/TiO2−x verify that the ESR reaction is boosted at the bimetal-support interface sites, in well agreement with the catalytic evaluations."   Figure 14 are great. When they are correlated wrongly with the elementary reactions, conclusion can mislead readers.

2) Experimental measurements shown in supplementary
Therefore, a link between the observations and the DFT results needs to be established carefully so that the results of the work are impactful.
Author reply: Thank you for this comment. According to this suggestion, we have improved the correlations between experimental observations and DFT calculation results in the revised manuscript.
• Page 8, Line 17: rephrase: "Furthermore, we performed kinetic tests on ethanol dehydrogenation, acetaldehyde decomposition, steam reforming of CO (or CH4) to study the C−H bond cleavage, C−C bond cleavage, CO or CHx transformation during ESR reaction. As shown in Supplementary Fig. 14a, the apparent activation energy of these reaction processes gives the following sequence: ethanol dehydrogenation (44.54 kJ mol -1 ) < acetaldehyde decomposition (50.45 kJ mol -1 ) < CO steam reforming (89.46 kJ mol -1 ) < CH4 steam reforming (101.26 kJ mol -1 ), which indicates that the cleavage of C−H and C−C bonds in ethanol is facile whilst the transformation of intermediates (CO and CHx) is rather difficult. This result is further demonstrated through a more significantly concentration-dependent reaction order for CO and